Segment Routing (SR) is an emerging technology which may be used in Multiprotocol Label Switching (MPLS) or Internet Protocol (IP) packet networks and which may be used in both a distributed control plane environment (e.g. an IP/MPLS control plane) and in a centralised control plane environment (e.g. SDN, Software Defined Networking).
Segment Routing, as defined by the Internet Engineering Task Force (IETF), enables enhanced packet routing, offering the ability to provide strict network performance guarantees whilst making more efficient use of network resources and providing greater scalability than other label-switched routing technologies which require a signalling protocol such as Resource Reservation Protocol-Traffic Engineering (RSVP-TE) to preconfigure end-to-end paths for data packets.
Segment Routing is based on two main concepts: Node Segment and Adjacency Segment. FIG. 1 shows an example network 10 with nodes A-G and Z. Each node in the network 10 is assigned a globally unique Segment Identifier (SID), called a node-SID. When a packet is injected at node A of the network with the segment identifier of node Z, nodes B-G will forward the packet to node Z via a shortest path which has been computed by the nodes at the time node Z was advertised to them. This is an unconstrained shortest path. In other examples, the node-SID is a locally unique identifier. In Adjacency Segment, an adjacency segment identifier, called an Adj-SID, identifies a particular link from a node. In FIG. 1, nodes B and C are connected by links with segment identifiers α and β. The adjacency SID can be a globally unique identifier or a locally unique identifier. If the adjacency SID is globally unique, the adjacency SID alone is enough to identify the link. If the adjacency SID is locally unique, then a combination of a node-SID and adjacency SID is required to identify the link.
A path computation entity (e.g. a PCE or an SDN controller) determines an end-to-end path for data packets across the packet network, and encodes this path as a set of ordered labels. Each label, as explained above, identifies a segment of the path. The set of labels is sent to the ingress network node. Each packet entering the network at ingress node A includes the set of labels in the header of the data packet. The ingress network node reads the outer, or uppermost, label and routes the data packets accordingly over the identified network segment to a further network node. The further network node drops the uppermost label, reads the next label and routes the data packets accordingly over the identified network segment and so on, until the data packets reach the egress network node.
A problem arises when a path is required to meet certain path constraints, such as total delay, bandwidth etc. The path computation entity can compute an end-to-end path which meets the required constraints. However, a row representation of the computed path as the sequence of all the node-SIDs/Adj-SIDs encountered along the path can require a large set of labels to define the hops of the path. Typically, routers in the network impose a limit on the maximum size of the label stack. Accordingly, it can be difficult to define a path which meets required constraints and yet has a label stack which is acceptable to equipment used at network nodes.